The present invention relates to steel cables (xe2x80x9csteel cordsxe2x80x9d) usable for reinforcing rubber articles such as tires. It relates more particularly to the cables referred to as xe2x80x9clayeredxe2x80x9d cables usable for reinforcing the crown reinforcement of radial tires.
Steel cables for tires, as a general rule, are formed of wires of perlitic (or ferro-perlitic) carbon steel, hereinafter referred to as xe2x80x9ccarbon steelxe2x80x9d, the carbon content of which is generally between 0.2% and 1.2%, the diameter of these wires generally being between 0.10 and 0.50 mm (millimetres). A very high tensile strength is required of these wires, generally greater than 2000 MPa, preferably greater than 2500 MPa, which is obtained owing to the structural hardening which occurs during the phase of work-hardening of the wires. These wires are then assembled in the form of cables or strands, which requires the steels used also to have sufficient ductility in torsion to withstand the various cabling operations.
For reinforcing radial tires, most frequently so-called xe2x80x9clayeredxe2x80x9d steel cables (xe2x80x9clayered cordsxe2x80x9d) or xe2x80x9cmulti-layerxe2x80x9d steel cables formed of a central core and one or more concentric layers of wires arranged around this core are used. These layered cables are preferred to the older xe2x80x9cstrandedxe2x80x9d cables (xe2x80x9cstrand cordsxe2x80x9d) owing firstly to a lower industrial cost price, and secondly to greater compactness, which makes it possible in particular to reduce the thickness of the rubberised plies used for the manufacture of tires. Among layered cables, a distinction is made in particular, in known manner, between compact-structured cables and cables having tubular or cylindrical layers.
Such layered cables, usable in particular for reinforcing radial tires, have been described in a very large number of publications. Reference may be made in particular to the documents GB-A-2 080 845; U.S. Pat. Nos. 3,922,841; 4,158,946; 4,488,587; EP-A-0 168 858; EP-A-0 176 139 or U.S. Pat. No. 4,651,513; EP-A-0 194 011; EP-A-0 260 556 or U.S. Pat. Nos. 4,756,151; 4,781,016; EP-A-0 362 570; EP-A-0 497 612 or U.S. Pat. No. 5,285,836; EP-A-0 567 334 or U.S. Pat. No. 5,661,965; EP-A-0 568 271; EP-A-0 648 891; EP-A-0 661 402 or U.S. Pat. No. 5,561,974; EP-A-0 669 421 or U.S. Pat. No. 5,595,057; EP-A-0 675 223; EP-A-0 709 236 or U.S. Pat. No. 5,836,145; EP-A-0 719 889 or U.S. Pat. No. 5,697,204; EP-A-0 744 490 or U.S. Pat. No. 5,806,296; EP-A-0 779 390 or U.S. Pat. No. 5,802,829; EP-A-0 834 613 or U.S. Pat. No. 6,102,095; WO98/41682; RD (Research Disclosure) No. 316107, August 1990, pp. 681; RD No. 34054, August 1992, pp. 624-33; RD No. 34370, November 1992, pp. 857-59; RD No. 34779, March 1993, pp. 213-214; RD No. 34984, May 1993, pp. 333-344; RD No. 36329, July 1994, pp. 359-365.
Among these layered cables, the most widely found in crown reinforcements for radial tires are essentially cables of formula [M+N] or [M+N+P], the latter generally being intended for the largest tires. These cables are formed in known manner of a core of M wire(s) surrounded by at least one layer of N wires which may in turn be surrounded by an outer layer of P wires, with generally M varying from 1 to 4, N varying from 3 to 12, P varying from 8 to 20 if applicable, the whole possibly being wrapped by an external wrapping wire wound in a helix around the last layer.
In order to fulfil their function of reinforcing crown reinforcements for radial tires, the layered cables must first of all have a high compressive strength, which involves in particular their wires, at the very least for the majority thereof, having a relatively large diameter, generally at least equal to 0.25 mm, higher in particular than that of the wires used in conventional cables for carcass reinforcements for tires.
It is important on the other hand for these cables to be impregnated as much as possible by the rubber, and for this material to penetrate into all the spaces between the wires constituting the cables, because if this penetration is insufficient, there then form empty channels along the cables, and the corrosive agents, for example water, which are likely to penetrate into the tires for example as a result of cuts or other attack on the crown of the tire, move along these channels across the crown reinforcement of the tire. The presence of this moisture plays an important part in causing corrosion and in accelerating the fatigue processes (so-called xe2x80x9cfatigue-corrosionxe2x80x9d phenomena), compared with use in a dry atmosphere.
Thus, in order to improve the endurance of the layered cables in the reinforcement armatures of the tires, it has for a long time been proposed to modify their construction in order to increase in particular their ability to be penetrated by rubber, and thus to limit the risks due to corrosion and to fatigue-corrosion.
There have for example been proposed or described layered cables of the construction [3+9] or [3+9+15] which are formed of a core of 3 wires surrounded by a first layer of 9 wires and if applicable a second layer of 15 wires, as described, for example, in EP-A-0 168 858, EP-A-0 176 139, EP-A-0 497 612, EP-A-0 568 271, EP-A-0 669 421, EP-A-0 709 236, EP-A-0 744 490, EP-A-0 779 390, EP-A-0 834 613, RD No. 34984, May 1993, pp. 333-344, the diameter of the wires of the core being or not being greater than that of the wires of the other layers. It is know that these cables cannot be penetrated by rubber. A channel or capillary remains at the center of the three core wires, which remains empty after impregnation by the rubber, and therefore favourable to the propagation of corrosive media such as water.
The publication RD No. 34370, in order to solve this problem, proposes cables of structure [1+6+12], of the compact type or of the type having concentric tubular layers, formed of a core formed of a single wire, surrounded by an intermediate layer of 6 wires which itself is surrounded by an outer layer of 12 wires. The ability to be penetrated by rubber can be improved by using diameters of wires which differ from one layer to the other, or even within one and the same layer. Cables of construction [1+6+12], the penetration ability of which is improved owing to appropriate selection of the diameters of the wires, in particular to the use of a core wire of larger diameter, have also been described, for example in EP-A-0 648 891 or WO98/41682.
In order to improve the penetration of the rubber into the cables, there have also been proposed or described multi-layer cables having a central core surrounded by at least two concentric layers, in particular cables of the formula [1+N+P] (for example [1+4+P] or [1+5+P]) or even [2+N+P] (for example [2+5+P]), the outer layer of which is unsaturated (i.e. incomplete), thus ensuring better ability to be penetrated by rubber (see, for example, RD No. 316107, August 1990, p. 681; EP-A-0 567 334 or U.S. Pat. No. 5,661,965; EP-A-0 661 402 or U.S. Pat. No. 5,561,974; EP-A-0 675 223).
Experience shows, however, that these cables having improved penetration ability have not, for the most part, yet been penetrated to the centre by the rubber, and in any case do not provide optimum performance in a tire.
It should in fact be noted that an improvement in the ability to be penetrated by rubber is not sufficient to ensure an optimum level of performance. When they are used for reinforcing crown reinforcements of tires, the cables must not only resist corrosion, but also must fulfil a large number of other, sometimes contradictory, criteria, in particular of tenacity, high degree of adhesion to rubber, uniformity, flexibility, resistance to impact and perforation, endurance under compression and under flexion-compression, all in a more or less corrosive atmosphere.
Thus, for all the reasons set forth previously, and despite the various recent improvements which have been made here or there on such and such a given criterion, the best cables used today in crown reinforcements for radial tires, intended in particular for heavy vehicles, remain limited to a small number of layered cables of highly conventional structure, of the compact type or the type having cylindrical layers, with a saturated (i.e. complete) outer layer; these are essentially cables of constructions [3+9] and particularly [3+9+15] as described previously.
The applicants during their research discovered a novel layered cable, of the type [M+N+P] having an unsaturated outer layer (with N equal to 4 or 5), which, owing to a specific structure, has not only excellent ability to be penetrated by rubber, limiting the problems of corrosion, but also increased endurance under compression. The longevity of the tires and that of their crown reinforcements is thus improved.
Consequently, a first subject of the invention is a multi-layer cable having a unsaturated outer layer, which can be used as a reinforcement element for a crown reinforcement of a tire, comprising a core (C0) of diameter d0 surrounded by an intermediate layer (C1) of four or five wires (N=4 or 5) of diameter d1 wound together in a helix at a pitch p1, this layer C1 itself being surrounded by an outer layer (C2) of P wires of diameter d2 wound together in a helix at a pitch p2, P being less by 1 to 3 than the maximum number Pmax of wires which can be wound in one layer about the layer C1, this cable being characterised in that it has the following characteristics (d0, d1, d2, p1 and p2 in mm):
(i) 0.10xe2x89xa6d0 less than 0.50;
(ii) 0.25xe2x89xa6d1 less than 0.40;
(iii) 0.25xe2x89xa6d2 less than 0.40;
(iv) for N=4: 0.40 less than (d0/d1) less than 0.80;
for N=5: 0.70 less than (d0/d1) less than 1.10;
(v) 4.8xcfx80(d0+d1) less than p1 less than p2 less than 5.6xcfx80(d0+2d1+d2);
(vi) the wires of layers C1 and C2 are wound in the same direction of twist.
The invention also relates to the use of a cable according to the invention for reinforcing articles or semi-finished products made of plastics material and/or of rubber, for example plies, tubes, belts, conveyor belts and tires, more particularly radial tires which use a metal crown reinforcement.
The cable of the invention is very particularly intended to be used as a reinforcing element for the crown reinforcements of radial tires intended for industrial vehicles selected from among vans, xe2x80x9cheavy vehiclesxe2x80x9dxe2x80x94i.e. subway trains, buses, road transport machinery (lorries, tractors, trailers), off-road vehiclesxe2x80x94agricultural machinery or construction machinery, aircraft, and other transport or handling vehicles.
The invention furthermore relates to these articles or semi-finished products made of plastics material and/or rubber themselves when they are reinforced by a cable according to the invention, in particular tires intended for the vehicles mentioned above, and also to composite fabrics comprising a matrix of rubber composition reinforced with a cable according to the invention, usable in particular as a crown reinforcement ply for such tires.